This invention relates to an electron lens, such as is used for example in an electron microscope.
It has long been recognized that electron lenses of the prior art have an undesirable degree of spherical aberration. The book "The World of the Electron Microscope" by Ralph W. G. Wyckoff (Yale University Press, 1958) says on page 27 "The defect which is at present the most important factor in limiting the attainable resolving power is spherical aberration." And later on the same page, "The only way to reduce this aberration is by keeping the opening of the lens small, but with too much aperturing diffraction effects would become damaging."
Fleming U.S. Pat. No. 2,740,919, granted Apr. 3, 1956, attempted to overcome the problem of spherical aberration in electron lenses, but was apparently not successful. Spherical aberration is considered at some length and some mathematical equations are given on pages 69-74 of the book "The Scanning Electron Microscope, Part I, The Instrument" by C. W. Oatley, published by the Cambridge University Press in 1972.
In view of the attention that has been given to the problem of spherical aberration in electron lenses, it can been seen that this is an important problem and that a satisfactory solution is much to be desired.
An object of the present invention is to provide an electron lens in which spherical aberration is eliminated or reduced to an acceptable degree.
Another object of the invention is to provide a practical method or process for making such an electron lens.